1. Field of the Invention
The present invention relates to a display device, and particularly, to a system for controlling driving lamp of backlight unit.
2. Description of the Related Art
Recently, display devices have been becoming more important as an information transfer media in the growing information society. Among those display devices, the liquid crystal display device has been rapidly replacing a cathode ray tube (CRT) as a next generation of display device. The liquid crystal display device has superior visibility, lower power consumption, and a clearer image quality.
In general, a liquid crystal display device includes a liquid crystal display panel having a thin film transistor array substrate attached to a color filter substrate with a uniform cell-gap therebetween and liquid crystal molecules in the uniform cell-gap, a driving unit for driving the liquid crystal display panel, and a back-light unit for supplying light to the liquid crystal display panel. Because the liquid crystal display device does not emit light by itself, a separate light source, such as a backlight unit, is installed at a rear surface of the liquid crystal display panel. A plurality of gate lines and data lines perpendicularly cross each other in the liquid crystal display panel to define pixels that are arranged in a matrix form. The light supplied from the separate light source is controlled by each of the pixels to produce an image.
A back-light unit includes a lamp for emitting light in response to receiving power; an inverter for adjusting an amount of light emitted from the lamp by controlling current of the power supplied to the lamp, and an optical member for uniformly distributing the light across the entire surface of the liquid crystal display panel by condensing and diffusing the light of the lamp. The back-light unit can be an edge-type having the lamp provided at a side surface of the liquid crystal display panel or an orthogonal-type having a plurality of lamps arranged at a rear surface of the liquid crystal display panel. The use of these types of backlights depends on the dimension of the liquid crystal display panel. For example, in a large liquid crystal display panel, the orthogonal-type is generally used such that a plurality of lamps are arranged at the rear surface of the entire surface of the liquid crystal display panel to provide high brightness, thereby supplying a large amount of light to the liquid crystal display panel.
FIG. 1 is a block diagram illustrating the related art liquid crystal display device. Referring to FIG. 1, a liquid crystal display device includes a timing controller 2 for generating a plurality of control signals in response to receiving image data DATA, a liquid crystal display panel 1 in which a plurality of data lines D1 to Dm and gate lines G1 to Gn cross each other, a data driving unit 4 for supplying image information to the liquid crystal display panel 1 in response to control signals from the timing controller 2, a gate driving unit 6 for applying a scanning signal to the liquid crystal display panel 1 in response to control signals from the timing controller 2, and an inverter 8 for controlling power to the lamp 10 in response to a control signal from the timing controller 2.
The timing controller 2 receives the image data DATA from an exterior graphic system, such as a graphic card, to generate a plurality of control signals for controlling the data driving unit 4 and the gate driving unit 6 using a basic control signal and image information included in the image data DATA. Afterwards, the timing controller 2 supplies the generated control signals to the corresponding data driving unit 4 and the gate driving unit 6. The timing controller 2 also supplies control signals to both the data driving unit 4 and the gate driving unit 6 to regulate driving timing.
In the liquid crystal display panel, gate lines G1 to Gn and data lines D1 to Dm cross each other to define a plurality of pixels arranged in a matrix configuration. The pixels are electrically connected to the data lines D1 to Dm in response to signals on the gate lines G1 to Gn. Accordingly, the pixels are driven in lines with image information of the data driving unit 4 transferred through the data lines D1 to Dm and a scanning signal of the gate driving unit 6 transferred through the gate lines G1 to Gn. The inverter 8 converts direct current power applied from the exterior into a high voltage that is supplied to the lamp 10. The inverter 8 also adjusts the amount of light from the lamp 10 by controlling the amount of current in response to a lamp control signal DCS1, which is applied from the timing controller 2.
An orthogonal-type backlight unit in which a plurality of lamps are typically arranged at a rear surface of the liquid crystal display panel 1 at a certain interval is generally used as the lamp 10. The orthogonal-type can advantageously provide images of high brightness by uniformly supplying a large amount of light across the entire surface of the liquid crystal display panel 1. However, by using the orthogonal-type backlight unit, the wider the liquid crystal display panel is, the more the number of lamps increases. As a result, power consumption proportional increases in as much as the number of lamps increases. Accordingly, research has been carried out with respect to methods for reducing power consumed in an orthogonal-type back-light unit.
A scanning driving method has been proposed to reduce power consumption. In the scanning driving method, not all of the lamps of a backlight unit are turned on. Only a certain number of lamps are turned on to display an image. According to this method, power consumption can be reduced by controlling the number of lamps that are simultaneously turned on.
FIG. 2 is an exemplary view illustrating lamps which are turned on and off according to the related art scanning driving method. Referring to FIG. 2, a driving frequency is 60 Hz, which is usually used, and accordingly one frame is 16.7 ms. The liquid crystal display panel 1 has eight lamps in a backlight unit. In FIG. 2, a white lamp indicates a light-on lamp and a black lamp indicates a light-off lamp.
A scanning signal is sequentially applied from the gate driving unit to the plurality of gate lines arranged in a horizontal direction on the liquid crystal display panel 1. Therefore, pixels arranged in a matrix configuration on the liquid crystal display panel 1 sequentially display images by lines corresponding to the gate lines. The lamps 10 arranged in the same direction as the gate lines turn on in correspondence with the plurality of gate lines, respectively. That is, the scanning signal is sequentially applied to the plurality of gate lines. While the pixels display images by a line, the lamps 10 corresponding to each position of the gate lines maintain their on-state. Thus, light is supplied to corresponding pixels in a line with lighting (turning on) of the lamps 10 sequentially one by one from an upper region of the liquid crystal display panel 1.
In the scanning driving method, since there is a maximum predetermined number of lamps to be turned on, after completely turning on the lamps in as many as the maximum predetermined number, when a new lamp 10 is turned on, the lamp 10 which had first been previously lit up is turned off so as to uniformly maintain the maximum predetermined number of light-on lamps 10. That is, as illustrated in FIG. 2, the lamps are turned on sequentially from a first lamp 10 until a maximum predetermine number of five lamps 10, for example, are turned on in a first frame. When a sixth lamp 10 is turned on, the first lamp 10 which had been first turned on is turned off. Thus, only five lamps 10 are in a light-on state, and the positions of where the lamps are turned on are shifted one by one. As a result, when the last lamp is turned on, the first frame is completed. In a second frame, the maximum predetermine number of five lamps 10, for example, are in their light-on state, and in the last light-on state of the lamps 10 in the first frame, the light-on position of the lamps 10 in the second frame is shifted as many as one lamp 10. That is, the first lamp 10 is turned on again, and the fifth to eighth lamps 10 maintain their light-on state.
FIG. 2 illustrates driving the lamps 10 during one frame. Here, a time for maintaining a light-on state of a specific lamp 10 with respect to one frame is referred to as a scanning duty, namely, a light-on time/frame of the specific lamp 10. For instance, the first lamp 10 maintains its light-on state until the fifth lamp 10 is turned on, but is turned off when the sixth lamp 10 is turned on. At this time, the first lamp 10 maintains its light-on state during 2.09 ms×5 of one frame (16.7 ms). In other words, five eighths of one frame is the scanning duty.
According to the scanning driving method, although each lamp 10 is turned on depending on the scanning duty, the maximum predetermine number of five lamps 10, for example, have a light-on state during each 2.09 ms, a unit section of one frame. Therefore, power consumption of the lamps 10 can be reduced and images can still somewhat be provided. However, compared with the related art method in which the lamps are continuously turned on during one frame, brightness of the images is less than when using the scanning driving method. Current applied to the lamps 10 is controlled by an inverter. In the lamp scanning method shown in FIG. 2, the inverter controls the current applied to each lamp according to the scanning duty, thereby adjusting an amount of light from all of the lamps.
FIG. 3 is an exemplary view illustrating the related art waveform of a current applied to each lamp according to the scanning duty. As illustrated in FIG. 3, an alternating current with a certain level is supplied to lamps during a predetermined scanning duty. That is, an inverter controls the current amount supplied to each lamp in every frame according to the scanning duty. However, because the amplitude of the current supplied to each lamp is fixed, scanning duty has to be reduced to reduce power consumption of the lamps. In other words, the time for supplying the current to each lamp is reduced, which results in a reduced amount of power being supplied to the lamp. As a result, brightness of the lamp may also be reduced. Thus, an average brightness can be changed due to a variation in the scanning duty, which results in a degradation of image quality in a liquid crystal display device.
When amplitude of the current is fixed at a high value to prevent a degradation of brightness of the lamp due to variation in the scanning duty, a large amount of power is supplied to the lamps. A large amount of power can cause a lack of uniformity of mercury densities within a lamp. Thus, a nonniformity of brightness may occur or an error in light color emitted from the lamp may occur, such as pink discharge. Therefore, reliability of the lamp may be lowered.